Ultrahigh-strength twip steel sheet and manufacturing method thereof

ABSTRACT

The present invention provides a TWIP steel sheet that has an austenite matrix structure and includes 15-25 wt % of manganese. The TWIP steel sheet has a recrystallization texture in which brass orientation is suppressed and {3 5 2} &lt;2 2 1&gt; is developed as its main orientation during heat treatment subsequent to cold rolling, and has an average plastic strain ratio of 1.2 or more, preferably 1.5 or more. Since this TWIP steel sheet has high workability and strength, complicated vehicle body components can be easily manufactured through a press molding process without cracking or rupturing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims under 35 U.S.C. §119(a) priority to Korean Application No. 10-2008-0079405, filed on Aug. 13, 2008, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present invention relates to a twinning induced plasticity (TWIP) steel sheet in which both slip and twin serve as a deformation mechanism at the time of plastic deformation, and a method of manufacturing the same.

2. Background Art

Generally, a high-tension steel sheet which is widely used as a material for vehicle body components has a tensile strength of 590˜780 MPa, a yield strength of 270˜350 MPa, an elongation rate of 25˜35% and a plastic strain ratio of 0.9˜1.2.

To take into consideration of many problems, such as rupturing, corrugating, and the like, caused by insufficient elongation rate at the time of press molding and to meet the demand of high strength of vehicle body components, a steel sheet is required to be thick to a certain degree.

Further, even though elongation rate is sufficiently ensured, it is generally difficult to form a steel sheet into vehicle body components because the vehicle body components are complicated and multi-functionalized. Therefore, it is required to greatly increase the plastic strain ratio of a steel sheet with the development of molding technologies.

In order to meet the above requirements, a ultrahigh-strength TWIP steel sheet, comprising: 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron was proposed (Korean Patent Application Publication No. 2007-0018416). However, the plastic strain ratio of the steel sheet still needs to be improved.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY OF DISCLOSURE

Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a ultrahigh-strength TWIP steel sheet having improved plastic strain ratio, and a method of manufacturing the same.

In order to accomplish the above object, the present invention provides a technology of remarkably improving the plastic strain ratio of a TWIP steel sheet under given conditions by controlling a recrystallization texture during annealing after cold rolling.

In one aspect, the present invention provides an ultrahigh-strength TWIP steel sheet having a recrystallization texture in which brass orientation is suppressed and {3 5 2}<2 2 1> is developed as its main orientation, and having an average plastic strain ratio of 1.2 or more, preferably 1.5 or more. This TWIP steel sheet includes 15˜25 wt % of manganese and has an austenite matrix structure.

The cold-rolled TWIP steel sheet can be manufactured as follows. A material composition is melted in an electric furnace and then continuously cast to obtain a slab, and then the obtained slab is hot-rolled at a temperature of 1300˜1100° C. Subsequently, the hot-rolled slab is gradually cooled to a temperature of 900˜600° C. at a cooling rate of 60° C./sec or less and then coiled to obtain a hot-rolled coil, and then the obtained hot-rolled coil is cold-rolled during 5˜7 passes at a reduction ratio of 20˜30% per pass.

In another aspect, a method for manufacturing the ultrahigh-strength TWIP steel sheet is provided.

As the present ultrahigh-strength TWIP steel sheets have a high average plastic strain ratio, they can be used to manufacture complicated vehicle body components in a simpler way through a press molding process without cracking and rupturing. Further, since the ultrahigh-strength TWIP steel sheets have high strength and moldability, they can satisfy the rigidity and workability necessary for vehicle body components while being in a decreased thickness, thus decreasing the weight of vehicles.

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

The above and other features of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the orientation distribution function (ODF) of a TWIP steel sheet according to a Comparative Example;

FIG. 2 is a graph showing the plastic strain ratio (R) to an angle relative to the rolling direction of a TWIP steel sheet according to a Comparative Example;

FIG. 3 is a graph showing the crystal grain boundary distribution to misorientation angle of a TWIP steel sheet according to a Comparative Example;

FIG. 4 is a graph showing the orientation distribution function (ODF) of a TWIP steel sheet according to an Example of the present invention;

FIG. 5 is a graph showing the plastic strain ratio (R) to an angle relative to the rolling direction of a TWIP steel sheet according to an Example of the present invention; and

FIG. 6 is a graph showing the crystal grain boundary distribution to misorientation angle of a TWIP steel sheet according to an Example of the present invention.

DETAILED DESCRIPTION

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

In one aspect, as discussed above, the present invention provides an ultrahigh-strength TWIP steel sheet having a recrystallization texture in which brass orientation is suppressed and {3 5 2}<2 2 1> is developed as its main orientation, and having an average plastic strain ratio of 1.2 or more, preferably 1.5 or more. This TWIP steel sheet includes 15˜25 wt % of manganese and has an austenite matrix structure.

Although this TWIP steel sheet may include a martensite or ferrite structure in a limited amount, it has generally a single austenite phase.

Most of natural or synthetic materials are polycrystalline materials in the form of lumps of crystals. Their crystallographic orientations are not disordered, and appear in specific orientations. These materials having ordered crystal orientations are referred to as materials having preferred orientations, that is, textures.

Generally, an austenite matrix metal plate, for example a TWIP steel sheet, has a crystallographic texture including copper orientation, Goss orientation, brass orientation, S orientation and cube crystal orientation. The relative volume fraction of the austenite matrix metal plate influences the average plastic strain ratio thereof. Meanwhile, the crystal orientation of the metal plate produced through rolling is defined by a rolling plane and a rolling direction. That is, the texture of the metal plate may be defined by a crystal plane placed parallel to the rolling plane and a crystal direction placed parallel to the rolling direction. Here, the crystal plane is represented by Miller indices {hk1}, and the crystal direction is represented by <uvw>. For example, copper orientation is represented by {112}<111>, Goss orientation is represented by {011}<100>, brass orientation is represented by {112}<110>, S orientation is represented by {123}<634>, and cube crystal orientation is represented by {001}<100>.

In a TWIP steel sheet, which is composed of a metal having low stacking fault energy (SFE), brass orientation is particularly developed during cold rolling. When this cold-rolled TWIP steel sheet is annealed at a temperature of 800˜900° C. for 8˜10 hours according to the prior art, so-called continuous recrystallization in which texture is hardly changed occurs, and annealing twin crystals (that is, Σ3 grain boundary) is increased. Meanwhile, this annealed TWIP steel sheet has an average plastic strain ratio of about 0.911, and it is difficult to increase the average plastic strain ratio thereof to a value higher than 0.911. Therefore, in the present invention, in order to increase a plastic strain ratio, brass orientation is suppressed, and {3 5 2}<2 2 1> is developed as a main orientation.

According to the present invention, a cold-rolled TWIP steel sheet is dually annealed. First, recovery heat treatment is conducted at a first temperature range such that, among high-angle grain boundaries, a coincide site lattice (CSL) grain boundary fraction, such as a Σ3 grain boundary fraction in which recrystallization does not easily occur, is decreased. Second, recrystallization heat treatment is conducted at a second temperature range such that the cold-rolled TWIP steel sheet is primarily recrystallized. In this annealed cold-rolled TWIP steel sheet, brass orientation is suppressed, {3 5 2}<2 2 1> is developed as the main orientation, and annealing twin crystals are remarkably decreased compared to the conventional cold-rolled TWIP steel sheets. Moreover, in this annealed cold-rolled TWIP steel sheet, the fraction of Σ3 grain boundaries having a misorientation angle of 60° may be 10% or less, the fraction of Σ3 grain boundaries having a misorientation angle of 5° or less may be 50% or more, and the fraction of Σ3 grain boundaries having a misorientation angle of 15° or more may be less than 40%.

The cold-rolled TWIP steel sheet may include 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron, as disclosed in Korean Patent Application Publication No. 2007-0018416.

As the reason why the amounts of the components are limited as specified above is well described in Korean Patent Application Publication No. 2007-0018416, the disclosure of which is incorporated herein by reference in its entirety, detailed description about the reason is omitted.

Preferably, the recovery heat treatment is conducted at a temperature of 230˜280° C. for 20˜30 minutes, and the recrystallization heat treatment is conducted at a temperature of 700˜820° C. for 6˜8 hours.

The cold-rolled TWIP steel sheet may be manufactured as follows. A material composition is melted in an electric furnace and then continuously cast to obtain a slab, and then the obtained slab is hot-rolled at a temperature of 1300˜1100° C. Subsequently, the hot-rolled slab is gradually cooled to a temperature of 900˜600° C. at a cooling rate of 60° C./sec or less and then coiled to obtain a hot-rolled coil, and then the obtained hot-rolled coil is cold-rolled during 5˜7 passes at a reduction ratio of 20˜30% per pass.

EXAMPLES

The following examples illustrate the invention and are not intended to limit the same.

In order to evaluate the material properties of the ultrahigh-strength TWIP steel sheet according to the present invention, a cold-rolled TWIP steel sheet having a composition shown in Table 1 below was annealed under various conditions, and then a test was conducted for the purpose of measuring the average plastic strain ratio thereof.

TABLE 1 Component C Si Mn Al P S Fe Amount 0.22 0.03 21 2.0 0.01 0.001 Residue

indicates data missing or illegible when filed

The cold-rolled TWIP steel sheet used in the test was manufactured as follows. The composition shown in Table 1 was melted in an electric furnace and then continuously cast to obtain a slab, and then the obtained slab was hot-rolled from 1300° C. to 1100° C. Subsequently, the hot-rolled slab was gradually cooled to a temperature of 900˜600° C. at a cooling rate of 40° C./sec and then coiled to obtain a hot-rolled coil, and then the obtained hot-rolled coil was cold-rolled during 7 passes. The cold rolling of the hot-rolled coil was conducted under plane strain deformation conditions at a reduction ratio of 30% or less per pass.

The annealing conditions of the cold-rolled TWIP steel sheet manufactured as above are given in Tables 2-1 and 2-2 below. As shown in Table 2-1, in Examples 1 to 9 according to the present invention, the cold-rolled TWIP steel sheet was dually annealed through recovery heat treatment and recrystallization heat treatment. Specifically, in Examples 1 to 9, the recovery heat treatment was conducted at a temperature of 230˜280° C. for 30 minutes, and the recrystallization heat treatment was conducted at a temperature of 700˜820° C. for 8 hours. As shown in Table 2-2, in Comparative Examples 1 to 9, the cold-rolled TWIP steel sheet was annealed through a conventional annealing process. In Comparative Examples 10 to 25, the cold-rolled TWIP steel sheet was dually annealed as in Examples 1 to 9 except that the heat treatment temperatures and times in Comparative Examples 10 to 25 was different from those in Examples 1 to 9. Specifically, in Comparative Examples 10 to 25, the recovery heat treatment was conducted at a temperature of 230˜280° C. for 5 or 40 minutes, and the recrystallization heat treatment was conducted at a temperature of 650 or 900° C. for 7 or 9 hours. Each of the recovery heat treatment and recrystallization heat treatment was conducted in a batch furnace.

TABLE 2-1 Recovery Recrystallization Average Class. Temp. (° C.) Time (min) Temp. (° C.) Time (h) R value Exp. 1 230 30 700 8 1.61 Exp. 2 230 30 770 8 1.71 Exp. 3 230 30 820 8 1.74 Exp. 4 250 30 700 8 1.62 Exp. 5 250 30 770 8 1.68 Exp. 6 250 30 820 8 1.68 Exp. 7 280 30 700 8 1.74 Exp. 8 280 30 770 8 1.75 Exp. 9 280 30 820 8 1.88

TABLE 2-2 Recovery Time Recrystallization Average Class. Temp. (° C.) (min) Temp. (° C.) Time (h) R value Comp. Exp. 1 — — 800 8 0.83 Comp. Exp. 2 — — 800 9 0.83 Comp. Exp. 3 — — 800 10 0.81 Comp. Exp. 4 — — 850 8 0.90 Comp. Exp. 5 — — 850 9 0.89 Comp. Exp. 6 — — 850 10 0.89 Comp. Exp. 7 — — 900 8 0.91 Comp. Exp. 8 — — 900 9 0.88 Comp. Exp. 9 — — 900 10 0.88 Comp. Exp. 10 230 5 650 7 0.77 Comp. Exp. 11 230 5 650 9 0.77 Comp. Exp. 12 230 40 650 7 0.81 Comp. Exp. 13 230 40 650 9 0.80 Comp. Exp. 14 230 5 900 7 0.78 Comp. Exp. 15 230 5 900 9 0.78 Comp. Exp. 16 230 40 900 7 0.66 Comp. Exp. 17 230 40 900 9 0.64 Comp. Exp. 18 280 5 650 7 0.83 Comp. Exp. 19 280 5 650 9 0.78 Comp. Exp. 20 280 40 650 7 0.86 Comp. Exp. 21 280 40 650 9 0.81 Comp. Exp. 22 280 5 900 7 0.82 Comp. Exp. 23 280 5 900 9 0.76 Comp. Exp. 24 280 40 900 7 0.73 Comp. Exp. 25 280 40 900 9 0.71

The TWIP steel sheets annealed under the annealing conditions shown in Tables 2-1 and 2-2 were tensile-strained at a tensile strain rate of 50% based on Korean standards (KS), and then the average plastic strain ratios (R values) thereof were calculated. These average plastic strain ratios were calculated by measuring plastic stain ratios in directions of respective angles of 0°, 45° and 90° relative to the rolling direction. As shown in Tables 2-1 and 2-2, all of the average plastic strain ratios (R values) of the TWIP steel sheets in Examples 1 to 9 were 1.5 or more. In contrast, none of the average plastic strain ratios (R values) of the TWIP steel sheets in Comparative Examples 1 to 25 exceeded 1.0.

Hereinafter, Examples will be compared with Comparative Examples with reference to FIGS. 1 to 6. FIGS. 1 to 3 are graphs showing the orientation distribution functions (ODFs), plastic strain ratios (R values) to angles relative to the rolling directions and crystal grain boundary distributions to misorientation angles of the TWIP steel sheet in Comparative Example 7, respectively. FIGS. 4 to 6, corresponding to FIGS. 1 to 3, are graphs showing the orientation distribution functions (ODFs), plastic strain ratios (R values) in directions of respective angles relative to rolling directions and crystal grain boundary distributions to misorientation angles of the TWIP steel sheet in Example 8, respectively.

As shown in FIG. 1, in the TWIP steel sheet of Comparative Example 7, brass orientation was developed as the main orientation, and as shown in FIG. 2, the average plastic strain ratio (R value) thereof was about 0.91, which did not exceed 1.0. Further, as a result of analysis of the TWIP steel sheet of Comparative Example 7 using an electron backscatter diffraction (EBSD) apparatus, as shown in FIG. 3, the density of this TWIP steel sheet was high near high-angle grain boundary, particularly, near a misorientation angle of 60° (Σ3 grain boundary). Here, the fraction of the Σ3 grain boundary was about 23%, and thus it can be seen that annealing twinning crystals were increased due to the increase in the fraction of the Σ3 grain boundary. Meanwhile, the fraction of the grain boundary having a misorientation angle of 15° or more was 81.2%.

As shown in FIG. 4, in the TWIP steel sheet of Example 8, brass orientation was remarkably decreased, and {3 5 2}<2 2 1> (20°, 70°, 30°) was developed as main orientation, and as shown in FIG. 5, the average plastic strain ratio (R value) thereof was about 1.754, which was increased to 190% or more of that of the TWIP steel sheet of Comparative Example 7. As shown in FIG. 6, the density of this TWIP steel sheet was remarkably decreased near a misorientation angle of 60°, and thus the fraction of the Σ3 grain boundary was about 8%. Here, it can be seen that annealing twinning crystals were also decreased due to the decrease in the fraction of the Σ3 grain boundary. Meanwhile, the fraction of the low-angle grain boundary having a misorientation angle of 5° or less was increased to about 52%, and the fraction of the grain boundary having a misorientation angle of 15° or more was about 36.1%, which is less than 40%. Interestingly, the grain boundary having a misorientation of more than 60°, appearing in Comparative Example 7, did not appear in Example 8 at all (refer to FIGS. 3 and 6).

According to the above results, it can be seen that the density of the Σ3 grain boundary was decreased through the recovery heat treatment, and the migration of residual high-angle grain boundary or the recrystallization of crystal grains was activated through the recrystallization heat treatment. As in Example 8, it is inferred that the annealing twin crystals are developed during subsequent processes, thus improving the workability of the TWIP steel sheet. To be brief, the development of texture of {3 5 2}<2 2 1> in the TWIP steel sheet of the present invention can be accomplished by dually performing the annealing treatment including recovery heat treatment and recrystallization heat treatment and by optimizing the annealing conditions including heat treatment temperatures and times.

As described above, although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

1. An ultrahigh-strength TWIP steel sheet having an austenite matrix structure comprising 15˜25 wt % of manganese, wherein the TWIP steel sheet has a recrystallization texture in which brass orientation is suppressed and {3 5 2} <2 2 1> is developed as a main orientation during heat treatment subsequent to cold rolling, and has an average plastic strain ratio of 1.2 or more.
 2. The ultrahigh-strength TWIP steel sheet according to claim 1, wherein the TWIP steel sheet comprises 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron.
 3. The ultrahigh-strength TWIP steel sheet according to claim 1, wherein a fraction of Σ3 grain boundaries having a misorientation angle of 60° is 10% or less, and a fraction of Σ3 grain boundaries having a misorientation angle of 5° or less is 50% or more.
 4. The ultrahigh-strength TWIP steel sheet according to claim 1, wherein a fraction of Σ3 grain boundaries having a misorientation angle of 15° or more is less than 40%.
 5. A method of manufacturing an ultrahigh-strength TWIP steel sheet having an improved plastic strain ratio, comprising: annealing a cold-rolled TWIP steel sheet that has an austenite matrix structure and comprises 15-25 wt % of manganese such that the cold-rolled TWIP steel sheet has a recrystallization texture in which brass orientation is suppressed and {3 5 2}<2 2 1> is developed as its main orientation, wherein the annealing of the cold-rolled TWIP steel sheet comprises: recovery heat treatment that is conducted at a first temperature range such that a fraction of a coincide site lattice (CSL) grain boundary is decreased; and recrystallization heat treatment that is conducted at a second temperature range such that the recrystallization texture is developed.
 6. The method of manufacturing an ultrahigh-strength TWIP steel sheet according to claim 5, wherein the cold-rolled TWIP steel sheet comprises 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron.
 7. The method of manufacturing an ultrahigh-strength TWIP steel sheet according to claim 5, wherein the recovery heat treatment is conducted at a temperature of 230˜280° C. for 20˜30 minutes.
 8. The method of manufacturing an ultrahigh-strength TWIP steel sheet according to claim 5, wherein the recrystallization heat treatment is conducted at a temperature of 700˜820° C. for 6˜8 hours. 